Electronic frequency modulator



Patented Feb. 8, 1949 ELECTRONIC FREQUENCY MODULATOR Amedeo D. Zappacosta, Havertown, Pa., assignor to Radio Corporation of America, a. corporation of Delaware Application January 31, 1946, Serial No. 644,526

'7 Claims. 1

The present invention relates to electronic frequency modulators and in particular to a method for and means of modulating the carrier frequency of an impulse type range indicator.

Range finders of the type employing a short pulse of radio frequency energy which is transmitted toward and reflected from a distant object are well known. Similar systems have been proposed with regard to underwater signalling for the location of submarines by means of supersonic impulses or for depth sounding to determine the distance between the surface of thesea and the sea floor. Systems of this general type may be improved by sweeping the carrier frequency from one predetermined value to another during each high frequency impulse. It is highly desirable in such systems that the starting frequency be the same for each of the successive impulses. Previously known methods of accomplishing this result have required a rotating capacitor in the oscillatory circuit of the carrier frequency generator, but difficulties arise in synchronizing the frequency modulator with the impulse period. Unless carefully synchronized each impulse will have a different starting frequency, depending upon the position of the capacitor at the instant the impulse is initiated.

In accordance with the present invention, a wholly electronic frequency modulating system is provided so that each of the successive impulses has the samefrequency at its beginning, and is then swept to a higher or lower frequency, as desired, in the interval corresponding to its duration. The present invention' also proposes a simplified electronic frequency modulator which is free from the inherent disadvantages of the previously known types in that it is D. C. operated and does not require phase shifting circuits.

It is therefore the primary object of this invention to provide an improved method of and means for electronically varying the frequency of a high frequency oscillator, and in particular to so modulate the carrier frequency of the impulse in an impulse type range finder as to insure uniformity as to the initial frequency of successive impulses.

A further object of this invention is to provide a frequency modulation system in which the frequency of an oscillator is controlled by varying the eflfect of a shunt or series connected reactance by means of a variable impedance device such as a vacuumtube. I

A still further object of this invention is to provide an improved system for varying automatically the carrier frequency of an impulse type range finder through a predetermined range at each impulse period.

A still further object -of this invention is to provide a system for insuring synchronism between the start of an impulse in an impulse type range finder and the frequency modulation of the carrier for each impulse cycle.

A still further object of this invention is to provide a system for varying the resonant frequency of a resonant reactance network by means of a thermionictube.

In accordance with one embodiment, the objects of the present invention are accomplished by connecting a reactance, such as a condenser, in parallel with the frequency determining circuit of the carrier frequency oscillator through the anode-cathode impedance of a thermionic tube. By varying the impedance of the tube by means of a D. C. bias applied to the grid, the reactance is effectively connected in parallel with the oscillatory circuit or it is disconnected therefrom with the result that the oscillatory frequency is caused to vary through a predetermined range.

The D. C. control bias is obtained from the os-' cillator grid circuit, which, as is well known, develops a positive bias when oscillations are started.

Other objects of this invention as well as a more complete understanding of its operation will be obtained from the following specification when considered in connection 'with the accompanying drawings, in which:

Figure 1 is a circuit diagram illustrating an electronic frequency modulator;

Figure 2 is a circuit diagram of an electronic frequency modulator utilized to produce voice frequencymodulations on a carrier frequency;

Figure 3 is a circuit diagram of a modified frequency modulator similar to that illustrated in Fig. 2; and

Figure 4 is a circuit diagram of a frequency modulated impulse oscillat-or'of the type employed in range finding apparatus.

Similar elements in the drawings will be in- I dicated by similar reference numerals.

Referring to Fig. 1, the tank circuit l represents the frequency determining network of an oscillatory circuit. not shown. Two capacitors, 3 and 5, are serially connected across the tank circuit. The anode-cathode path of a thermionic tube 1 is connected. in parallel with capacitor 5. -A

variab e grid bias is applied to a tube 1 by means.

of a potentiometer 9 and a battery II.

It is well known that the impedance of the anode-cathode path of a thermionic tube may be pedance across it or assume any intermediate value. Capacitor 5, in series withcapa'citor 3, is in the frequency determining circuit of the oscillator. When a negative grid voltage is applied to tube 1, the oscillatory frequency will be determined by the circuit constants including capacitors 3 and 5 in series, plus the small interelectrode capacity of the tube effectively in parallel with capacitor 5. When a positive bias is applied to tube 1, capacitor 5 is short circuited and the frequency is then determined by the circuit constants considering capacitor 3 as being effectively in parallel with the tank circuit I. It will be appreciated that the oscillatory frequency will therefore vary between two values as determined by the circuit constants in the two-limiting 'conditions described. For intermediate values of grid voltage, the oscillator will therefore have intermediate values of frequency, and frequency modulation of the oscillator may be effected by varying the applied D. C. grid voltage. The range of variation will be determined by the relative values of the capacitors 3 and 5 and their reactances with respect to the tank circuit 1.

Applying this principle to the voice modulation of an oscillator, a circuit such as is shown in Fig. 2 may be employed. Modulating voltage from any source may be applied to an input transformer i3, the secondary of which is connected between the cathode and grid electrodes of the modulating tube 1. The plate electrode is connected to the midpoint of capacitors 3 and 5, as in the case just discussed. The latter capacitors are serially connected across the tank circuit 1' of an oscillator l5. Frequency modulated output may be obtained in any desired manner as indicated by the output transformer 11 and coupled to a subsequent amplifier, antenna or other load.

The modulation voltage will change the impedance of tube 1 and thereby effect the frequency of oscillation for the reason discussed above.

If desired, the capacitor 5 may be eliminated and the frequency modulation effected by connecting a single capacitor 3 across the tank circuit i through the anode-cathode path of the modulated tube in the manner shown in Fig. 3. In this case it will be observed that in one limiting condition capacitor 3 is eifectively in parallel with the tank circuit, and the oscillatory frequency will have a low value. In the other limiting condition the capacitor 3 is disconnected from the circuit and the oscillatory frequency will have a maximum value.

A particularly useful application of this frequency modulating system is illustrated in Fig. 4,

to which reference is now made.

Tube I9 is a grounded-anode oscillator in which the grid and cathode electrodes are coupled to the oscillatory tank circuit l, in parallel with which are connected capacitors 3 and 5 in series. D. ,C. plate voltage is applied from a terminal 2| through a pulse keyer 23 and resistor 25, which may be bypassed to ground by a capacitor 21.

The pulse keyer 23 may be any mechanical or electronic device which connects and disconnects the source of positive potential to produce impulses of carrier frequency energy of the desired duration at the desired intervals. Output from the oscillator is obtained by a coupling capacitor 29 which applies the energy to the input of a power amplifier 3i, of any conventional design, the out- .put of which is radiated, for example, by means of an antenna 33.

The plate of the modulator tube 1 is connected to the midpoint of capacitors 3 and 5 and the cathode is returned to ground. The grid of the oscillator tube 19 is connected to the grid of the modulator tube 1 through an isolating resistor 35 and a resistance-capacitance filter network 31. Plate voltage for the modulator tube is supplied from the same source utilized to energize the oscillator, by means for coupling resistors 39 and M It is well known that a ositive bias is developed by the grid of an oscillapr tube when it is oscillating. At the instant the pulse keyer operates to apply plate voltage to the tube l9, oscillations begin. At this instant the grid bias of the oscillator tube is zero and therefore the bias of the modulator tube I is also zero. By employing a modulator tube of suitable characteristics, with zero grid bias the modulator presents a high impedance across capacitor 5 and the initial oscillation frequency is therefore determined by the circuit constants of the tank circuit I in parallel with capacitors 3 and 5. As oscillations develop,

a positive grid bias is produced on the grid of the oscillator tube which is applied likewise to the grid of the modulator causing it to present an increasingly lowered impedance across capacitor 5 and therefore switch the oscillator frequency from a high value to a low value. The purpose of filter 37 and isolating resistor 35 is to prevent the application of radio frequency energy to the grid of the modulator tube 1.

Since the impulse duration is extremely short, the pulse keyer 23 then opens the circuit and oscillations stop. The positive potential which had been developed on the grid of the oscillator tube l 9 now leaks oifthrough the grid leak resistor 43 and when the initial starting conditions have been reestablished, the second impulse repeats the process just described.

While I have shown my invention as a radio frequency impulse type range finder, it will be understood that my invention may also be equally applied in connection with underwater supersonic range finders operating on the same principle. In such a case the carrier frequency will be in the supersonic range and a magnetostrictive projector would normally be employed in place of antenna 33. Also it will be understood that the modulator tube may be connected in parallel with the tank circuit in place of capacitors 3 and 5. Alternatively, the modulator may be connected in parallel with a portion of the tank circuit reactance to produce the same result.

What I claim is:

'1. In a device of the character described, a reactance network, a thermionic oscillator tube coupled to said network for producing oscillatory currents and having a grid resistor which develo'ps a bias voltage when oscillations are initiated, a modulator tube including grid, cathode and anode electrodes and having its anode-cathode impedance eflectively in shunt with a portion of said network whereby changes in the cathodeanode impedance of said modulator eifect the frequency of said oscillatory currents and a conducting circuit connection with said oscillator grid resistor for applying said bias voltage to the grid electrode of said modulator tube.

2. The combination comprising an oscillator, a

keying circuit for establishing oil! and on periods of oscillation, a frequency modulating circuit coupled to said oscillator for varying the frequency thereof cyclically in accordance with a bias voltage, and circuit means for deriving said bias voltage from said oscillator.

3. In combination, an oscillator including a frequency determining circuit, a keying circuit for establishing cyclic off and on periods of oscillation, a resistance device variable under the control of a bias voltage," and coupled to said frequency determining circuit so. as to'modify the and thereby cause the. frequency of oscillation to sweep from one preselected value to another during each impulse. r

6. In an impulse generator, the method of varying the carrier frequency between predetermined limits during each impulse period which includes the steps of generating oscillations of a desired carrier frequency, periodically interrupting said oscillations to produceimpulses of a desired duration and continuing through a desired interval,

deriving a bias-voltage from said impulses, filtering said derived voltage to provide a controlpotential, and utilizingsaid control potential to vary the carrier frequency of said oscillations during each pulse.

7. The combination comprising an oscillation generator including a thermionic tube and a tuned efiective reactance of said circuit, and circuit oscillator.

4. The combination comprising a frequency determining resonant circuit, a thermionic tube coupled thereto to provide an oscillation generator, keying means for establishing cyclic 011" and on periods of oscillation, and modulating means means for deriving said bias voltage from said for sweeping the frequency of oscillation between predetermined limits once during each of said 011" periods, said means including resistance means variable under the control of a bias voltage, and means for developing a bias voltage in said generator and for applying said voltage to I said modulating means.

5. In a frequency modulated impulse generator I having an oscillator and a frequency determining resonant circuit, the method of operation which includes generating oscillations in. said resonant circuit, interrupting said oscillations cyclically to produce distinct impulses of short duration, deriving a bias voltage from said oscillator which corresponds to the duration of said impulses, varying the eflective resistance of a portion of said resonant circuit to control the frequency of said oscillations, and utilizing said bias voltage to control said resistance variation between limits erator.

AMEDEO D. ZAPPACOSTA.

REFERENCES CITED The following references are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date 2,256,539 Alford Sept. 23, 1941 2,341,655 Roberts Feb. 15, 1944 2,361,437 Trevor Oct, 31, 1944 2,383,848 Crosby Aug. 28, 1945 2,392,625 Usselman Jan. 8, 1946 2,407,644 Benioif Sept. 17, 1946 2,409,457 Usselman Oct.15, 1946 2,430,126 Korman Nov. 4, 1947 

